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Abstract – Multiply charged high�current ion beams of he�avy metals find wide technological applications. One way ofproducing ion beams is to extract ions from the vacuum arcplasma heated by high�power microwaves. A radial plasmainjection trap for heating the vacuum arc plasma by gyrationmicrowaves has been developed. A peculiar feature of thismethod of plasma injection into a trap is low longitudinal ve�locities of ions in the trap. This lengthens the time for whichions are confined in the trap and, hence, increases the ioncharge. This paper discusses the possibilities of additionalionization of the vacuum arc plasma of metals in the magnet�ic trap through electron�cyclotron resonance microwave he�ating and considers the parameters of the thus produced ionbeams.

1. Introduction

Multiply charged ion beams of heavy metals findapplication in solving a wide range of fundamental,for example physical experiments to synthesize newchemical elements of the periodic table and appliedproblems like ion modification of solid surfaces as anovel technological process of up�to�date industry.

In terms of the use of ion sources for solving ap�plied problems of surface modification, an increasein ion charge may provide a possibility of increasingthe ion energy without a proportional increase in ac�celerating voltage. It should be noted that the appro�ximate coast of an ion accelerator with ion energy ofhundreds of kiloelectronvolts and the X�ray dozepower produced by secondary electrons exhibit a qu�adratic dependence on the accelerating voltage, allother things being equal. In this connection, the op�timal way of increasing the ion energy without a inc�rease in accelerating voltage is to provide multiple io�nization in the plasma and, hence, a correspondingincrease in the charge of the ions extracted from thisplasma.

The use of an electron�cyclotron resonance(ECR) discharge in a magnetic trap in ion sourceshas made it possible to produce gas ions with reaso�

nably high current and high average charge [1]. Forthe generation of multiply charged ions of fusiblematters, oven can be used in which the working mat�ter is heated and evaporated [1]. Physically, the ECRbreakdown in metal vapors under these conditionsoccurs in much the same manner as that in gases. Ashortcoming of this method is that it does not allowgeneration of ions of refractory metals.

A vacuum arc discharge appears to be the mostappropriate way of producing ions of refractory me�tals [2]. Sources based on a vacuum arc can providegeneration of metal ion beams with currents of seve�ral amperes both in the pulsed and continuous mo�des [2]. In such a source, the average charge of ionsin the plasma is determined by the cathode materialand is 1.5–2.5. With special methods (such as super�position of a magnetic field on the near�cathode re�gion, application of additional arc discharge pulses,electron beam injection into the vacuum arc pla�sma), one can increase the charge of the ions, but inso doing the average ion charge increases no greaterthan 2.5 times [3, 4].

It is clear to use a vacuum arc discharge to injectthe plasma of refractory metals into an ECR sourcein order to further increase the ionization ratio ofmetal ions in the magnetic trap on microwave elec�tron heating. But the earlier such experiments usingmicrowaves of the centimeter range [5, 6], as is doneto increase the gas ion charge, have not a great resultof ion charge growing. In our viewpoint, this was dueto the fact that the confinement parameter Neτi (Ne isthe electron density and τi is the lifetime of ions inthe trap), which is responsible for the formation ofmultiply charged ions, was no greater than 108 cm–3s.The main reason for this was the small value of τi dueto high directional velocities of ions of the vacuumarc plasma [9]. The use of a gyrotron with higherpower (up to 100 kW) and short�wave radiation(8 mm) [7] has enabled a considerable progress inthis field.

Beam and plasma sources

Magnetic Trap for Multiple Ionization of Vacuum Arc Plasmas

by Electron�Cyclotron Resonance Heating1

A.G. Nikolaev1, E.M. Oks1,3, G.Yu. Yushkov1, V.I. Gushenets1, K.P. Savkin1,

Yu.G. Yushkov1, A.V. Vodopyanov2 and S.V. Golubev2

1High Current Electronics Institute of Siberian Division of Russian Academy Science, Tomsk, 634055,

Russia; Phone: +7�(3822)�491776, Fax: +7�(3822)�492410, GYushkov@opee.hcei.tsc.ru2Institute of Applied Physics of Russian Academy of Science, Nizhny Novgorod, Russia

3Tomsk State University of Control Systems and Radioelectronics, Tomsk, Russia

1 The work was supported by RFBR, Grant # 05�02�16256�a

2. Background

Problem of ECR heating of vacuum arc plasmawas solved collaboratively by research team repres�ented of two Institutes of Russian Academy of Scien�ces: Institute of Applied Physics (IAP) and Instituteof High Current Electronics (HCEI). Experimentalwere carried out jointly at the ECR test bench of IAP,where as a compact vacuum arc plasma generatorwas designed and made by HCEI research group. Jo�in efforts of both research groups provides first expe�rimental evidence of enhancement vacuum arc me�tal ion charge states by ECR heating [8]. Results offurther investigation in such direction were also pu�blished in details elsewhere [9].

The general arrangement of the setup used for jo�int ECR heating of vacuum arc plasma experimentsis shown in Fig. 1. Plasma generator 3 was arrangedon the system axis near one of the taps of a magnetictrap formed by two coils 6. The magnetic field in thetaps ranged to a few Tesla. Plasma generator powersupply 4 produced arc discharge pulses of current100 A and duration longer than 100 μs. Dischargechamber 5 of the magnetic trap had a teflon windowthrough which radiation produced by gyrotron 1 wasdelivered into the discharge region. The oscillationfrequency was 37.5 GHz, the power ranged to100 kW, and the pulse duration was 1.5 ms.

Fig. 1. Magnetic trap for ECR discharge heating of thevacuum arc plasma: 1 – gyrotron, 2 – dielectric lens,3 – vacuum arc plasma generator, 4 – power supply ofthe plasma generator, 5 – discharge chamber, 6 – co�ils of the magnetic field, 7 – ion spectrometer

The interaction of microwaves with plasma elec�trons under the ECR condition considerably incre�ased the electron energy that, in turn, led to additio�nal ionization of ions by electron impact. A maxi�mum average charge of 4.3 was obtained for plati�num ions at a vacuum arc current of 80 A, a magnet�ic field in the trap taps of 2.6 T and a microwavepower input from gyration of 60 kW. Example ofsuch increasing of ion charge state shows on Fig. 2.

Estimates show that the confinement parameterattained in the experiments Neτi=3.108 cm–3s closelycorresponds to the obtained maximum average ioncharge. The current density of an ion beam extrac�table from such plasma is 1 А/cm2. At this moment,the obtained experimental results prove to be record�breaking.

Fig. 2. Charge state distributions of platinum ionswith and without microwave heating

One of the main parameters responsible for theefficiency of generation of multiply charged ions inthe system under study is the velocity of ions of thevacuum arc discharge plasma, since it is this velocitythat affects the time for which ions are confined inthe trap and determines finally the value of the con�finement parameter. With a fixed size of the magnet�ic trap, the ion velocity specifies the time of interac�tion of hot electrons with metal ions and, hence, theionization ratio of these ions.

It is known that the magnetic field present in thecathode region of a vacuum arc discharge causes anincrease in discharge operating voltage and, consequ�ently, in the velocity of ions emitted by cathode spots[10]. Measurements show that the directional ion ve�locity increases considerably when the magnetic fieldrequired for the ECR discharge ignition is superimpo�sed on the discharge plasma which is due to the effectof the magnetic field on the operating discharge volta�ge. Because of this, the average lifetime of an ion inthe trap was no longer than 15 μs. On the other hand,simple estimates demonstrate that with a 60�kWpower input into the trap the useful power expended inincreasing the ion charge is no greater than 1 kW.

Thus, the key moment in further increasing theion charge by microwave heating of electrons in themagnetic trap with an ECR discharge is to increasethe lifetime of ions in the trap with increasing plasmaelectron density for the microwave power to be usedmore efficiently.

3. New magnetic trap with radial injection of metal plasma

To further increase the ion charge, a number ofmeasures have been taken to increase the degree ofheating of plasma electrons with simultaneous confi�nement of ions.

First, we increased the microwave power of thegyrotron and calculated the microwave channel ofthe new magnetic trap. At present, the microwave sy�

1 2 3 4 5 60,00,10,20,30,40,50,60,70,80,9

Ion

curr

ent,

arb.

units

Ion Charge State

Pt-cathode no microwave 63 kW of microwave

1 3

4

5 7

2 6 6

Beam and plasma sources

144

stem provides application of gyration microwavepulses with a power of up to 400 kW for 4 mm wave�radiation, a duration of up to 100 μs, and a pulse re�petition rate of up to 25 Hz into the discharge regionof the magnetic trap. Another change taken to confi�ne the plasma was the increase in the magnetic fieldof the trap taps to 5 T (Fig. 3). To this propose, a spe�cial water�cooled low�inductance system was desig�ned. The ordinary coil conductors were replaced by acopper tube with water running through. To increasethe microwave power density inside the dischargechamber and the magnetic field, the inner diameterof the discharge chamber of the trap was reduced to3.2 cm.

For the purpose of increasing the discharge pla�sma density and simultaneously the time of ion con�finement in the trap, it was proposed to inject the va�cuum arc plasma at the trap center with four plasmagenerator operating at the same time (Fig. 3). Theuse of this system of plasma injection into the trapmay remove the limitations placed on the one catho�de. In the previous design, the one cathode unit lo�cated on the trap axis and also its shape was chosenfrom matching the microwave channel of the trap tominimize the reflected power. The cathode units we�re standard units which are employed in ion sourcesof the Mevva type [11] and have a high stability ofoperation within no less than 106 pulses

The arrangement of four cathodes in the trapcenter at the periphery of the chamber�waveguideensures another method of heating the plasma. Theplasma supplied into the discharge region from foursides drifts around the trap tube in the electric andmagnetic fields, forming a "plasma sheath". In thecollisionless case, the thickness of the "plasma she�ath" must be equal to the Larmor diameter of ionswhich is estimated to be several millimeters and theconfiguration of the sheath itself must replicate theforce line distribution in the trap, i.e., have a largerdiameter in the injection region and a smaller diame�ter in the tap region. When arrived into the "plasmasheath", the microwave radiation provides heating ofelectrons to high temperatures, resulting in additio�nal ionization of the cathode material. The radiationflux will be "locked" inside the plasma sheath thatmust ensure more intense interaction of the mic�rowaves with the plasma

The radial injection of the vacuum arc plasmawherein high directional ion velocities are directed"crosswise" the trap length must lead to an increasein the lifetimes of ions in the trap. With the axial pla�sma injection used earlier, the longitudinal magneticfield of the trap caused an increase in ion velocityand a corresponding decrease in the time for whichthey are in the ionization region. Increasing themagnetic field in the system with radial injection in�volves magnetization of electrons and ions of the pla�sma, thus contributing to their confinement. So, in

the absence of a magnetic field the time of ion con�finement in the trap estimated from the shift of theion current pulses to the Langmuir probe at the trapface with respect to the discharge current pulses was~10 μs. The occurrence of a magnetic field increas�ed this time. The time in which the ions reached thecollector increased to over 30 μs even with relativelysmall magnetic fields of 1 T (Fig. 4).

Fig. 3. From top to bottom: picture of magnetic trapwith four�cathode unit at center, sketch of the trap,and magnetic field distribution to along of the trap

-20 -15 -10 -5 0 5 10 15 200

1

2

3

4

5

6

Mag

netic

fiel

d, T

Distance from cathodes, cm

Poster session

145

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146

Fig. 4. Vacuum arc current and ion current from trapwithout (top) and with (bottom) magnetic field. Arccurrents are 120 Amps per div. Ion currents are 2 mAper div. (top) and 1 A per div. (bottom)

Thus, it can be stated that the radial injection ofthe vacuum arc plasma increases the time for whichions reside in it at least threefold. Taking into ac�count that four vacuum arc discharge ignited at a ti�me ensure a high plasma density, the confinementparameter Neτi is expected to be greater than109 cm–3s–1 which, at high electron temperature, me�ans production of ions of heavy metals with a chargeof 10+ and over.

4. Conclusion

The increase in the charge of metal ions allows anincrease in ion beam energy without a correspondingincrease in accelerating voltage. This makes an ion

source cheaper and decreases the power of spuriousbremsstrahlung. The use of 100�kW gyrotron mic�rowave radiation has made it possible to generateions of heavy metals with a charge of up to 6+. Theproposed magnetic trap with radial injection of thearc discharge plasma has allowed us to produce ionbeams with a charge of 10+.

AcknowledgmentThe authors appreciate the support of the Russian

Foundation for Basic Research, Grant # 05�02�16256�a.

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